1. Field of the Invention
The present invention relates to a covering material for electric wires and, more particularly, to a polyester resin composition comprising a halogen-containing flame retardant aromatic polyester copolymer and a bisoxazoline compound in mixture therewith, which is not liable to loss of flexibility due to its thermal history and has excellent flame retarding properties, and to a covered electric wire using same.
2. Description of Related Art
Hitherto, rubber, polyvinyl chloride, polyethylene, polypropylene, nylon, and the like have been used as covering materials for electric wires, and above all, polyvinyl chloride has been largely used as a covering material from the view points of flame retardancy and mechanical strength. Recently, as environmental conditions become severer for use of these covering materials, higher performance characteristics have been required of such material, including, in addition to high heat resistance and good electrical properties, excellent flame retardancy and such a degree of thickness reducibility as is satisfactory enough to suit space saving purposes.
Fluoroplastics, crosslinked polyethylenes, and the like meet many of these requirements, but they have a disadvantage with respect to thickness reducibility. Further, fluoroplastics are expensive. As such, these resins cannot be considered to be satisfactory.
Much attention is being paid to polyethylene terephthalate and polybutylene terephthalate because they have good thickness reducibility, and further because they are well qualified with respect to mechanical strength characteristics (such as flexibility and wear resistance), heat resistance, and electrical properties. However, they are not well qualified in flame retardancy, and since these polyalkylene terephthalates are crystalline resins, they are liable to excessive deterioration in flexibility and impact strength and other mechanical strength characteristics as a consequence of their exposure to heat in the course of heat treatment after the process of wire covering and/or in the course of their being used under heat conditions. As such, it is necessary that their use at locations adjacent a source of heat or in environmental conditions which are likely to lead to heat accumulation should be avoided, and thus no small limitation is imposed upon the use of those resins as such.
With a view to overcoming such difficulty, an attempt has been made to reduce the crystallinity of such a resin even to a small degree by admixing an elastomer or a non-crystalline polymer. It has also been attempted to effect partial crosslinking so as to maintain the mechanical strength characteristics of such a resin.
Some improvement can be observed with the first mentioned attempt, but since the matrix of the crystalline resin remains as such, the resulting composition cannot withstand any long-time exposure to heat; and, a decrease in the proportion of the crystalline resin is likely to lead to some degradation in the mechanical properties of the composition, such as frictional wear characteristics.
With the second mentioned attempt, some improvement due to crosslinking can be found with respect to mechanical property stabilization, but since the property of flexibility is sacrificed and since a crosslinking reaction is involved, the process requires complicated control and the processability of the resulting composition is very adversely affected.